Rust E0597 — borrowed value does not live long enough

The invalid lifetime

A reference cannot outlive its owner. If a function tries to return a reference into a local String, that String is destroyed on return. Lifetime annotations do not create extra storage or prolong the underlying object.

// fn broken<'a>() -> &'a str {
//     let local = String::from("temporary");
//     &local // invalid reference to destroyed storage
// }

fn owned() -> String {
    String::from("persistent")
}
fn main() {
    let text = owned();
    let view: &str = &text;
    assert_eq!(view, "persistent");
}

Borrow from the caller instead

If the returned slice should refer to an existing argument, make that relationship explicit:

fn first_word(input: &str) -> &str {
    input.split_whitespace().next().unwrap_or("")
}

The returned &str is usable while the input remains alive. If the caller needs to keep the result after discarding the input, return an owned String instead.

Experiment

Call first_word on a long-lived String, then attempt to keep the borrowed result outside that String's scope. Read the compiler's lifetime diagnostic and change the API contract, not the annotation to a fabricated static lifetime. Repeat with a struct that must store either a borrowed view with a legitimate lifetime parameter or an owned String.

Reference: https://doc.rust-lang.org/error_codes/E0597.html

A lifetime is a relationship, not a memory allocator

For fn first_word(input: &str) -> &str, the returned value is a view inside input. Test first_word("") == "", first_word("Ada Lovelace") == "Ada" and first_word(" Alan") == "Alan". Now redesign a cache that must retain a word after the source buffer is dropped: return String with .to_owned(), rather than claiming &'static str. A literal string is static, but a slice into a caller-owned String is not. In a struct storing both a String and a reference into itself, a self-referential design requires special care; simpler ownership or stable indices are often safer than inventing lifetime parameters.